Electroplating solutions for depositing silver alloys and a method of forming silver alloys by electroplating

ABSTRACT

An iodide-containing electroplating solution for depositing alloys of silver with another metal such as copper, indium, zinc or mixtures thereof. The solution comprises a silver salt, an iodide and a salt of one or more of the above metals.

United States Patent [1 1 Yonezawa et al.

[451 Oct. 21, 1975 [541 ELECTROPLATING SOLUTIONS FOR DEPOSITING SILVER ALLOYS AND A METHOD OF FORMING SILVER ALLOYS BY ELECTROPLATING [75] Inventors: Kazuo Yonezawa; Kaname Nakao;

Takashi Suzuki; Satoshi Kaneda, all of Osaka, Japan [73] Assignee: Matsushita Electric Industrial Company, Kadoma, Japan [22] Filed: June 12, 1973 [21] Appl. No.: 369,196

[30] Foreign Application Priority Data June 16, 1972 Japan 47-60780 June 21, 1972 Japan..... 47-62633 Sept. 16, 1972 Japan 47-94826 [52] 11.5. Cl. 204/44; 75/153; 75/157; 75/173 R; 75/173 C; 204/43 R [51] Int. Cl. CZSD 3/58; C25D 3/64 [58] Field of Search 204/43 R, 46 R, 44

OTHER PUBLICATIONS The Canadian Patent Office Record, p. 765, Apr. 1, 1947.

Primary Examiner-G. L. Kaplan [57] ABSTRACT An iodide-containing electroplating solution for depositing alloys of silver with another metal such as copper, indium, zinc or mixtures thereof. The solution comprises a silver salt, an iodide and a salt of one or more of the above metals.

13 Claims, No Drawings ELECTROPLATING SOLUTIONS FOR DEPOSITING SILVER ALLOYS AND A METHOD OF FORMING SILVER ALLOYS BY ELECTROPLATING The present invention relates generally to an electroplating bath for codepositing silver alloys on objects with particular reference to an improved electroplating bath containing iodide to provide silver alloy coatings wherein silver is coprecipitated with one or more base metals.

The principal use of silver plating is tableware plating, because of the pleasing appearance of the plated object and the resistance to attack by most foods. Silver is also used for plating the surfaces of electrical contacts and the interior of waveguides, because of its low electrical resistance, Silver plating is done conventionally in an aqueous solution of silver cyanide. However, the disadvantage is that the conventional silver plating results in a poor resistance to humidity and sulfurous fumes, and hence, a potential source of contact failure in electric circuits. Because of such disadvantages costly gold plating is used for expensive articles and electronic parts of which high reliability is 'required.

On the other hand, plating with silver alloys known in the art involves the use of a silver cyanide bath in which articles to be electroplated are immersed. However, such plating with silver alloys is successful only if the coprecipitating compound is a precious metal such as gold. Therefore, it has so far been difficult to achieve satisfactory coprecipitation of base metals with silver. Furthermore, the disadvantage encountered with the conventional cyanide bath is that it is harmful to the human body requiring the greatest care to be taken in electroplating operation, and the waste materials would lead to the pollution of the environment.

It is, therefore, an object of the present invention to provide an improved electroplating bath containing iodide for coprecipitation of silver with one or more base metals.

Another object of the invention is to provide an iodide-containing electroplating bath to codeposit a silver alloy with copper, indium and/or zinc.

A further object of the invention is to provide a harmless electroplating bath for safety against the human body and for preventing waste materials which would pollute the environment.

Still anotherobject of the present invention is to provide an iodide-containing electroplating bath to provide a silver alloy coating wherein silver is coprecipitated in a ternary phase with any two of copper, indium and zinc.

Still another object of the invention is to provide an iodide containing electroplating bath to provide a silver alloy coating wherein silver is coprecipitated in a quaternary phase with copper, indium and zinc.

There is no theory generally applicable to the deposition of alloy coatings. In usual practice, alloy coatings are deposited either by forming complex compounds or by polarizing one of the depositing metals. According to the present invention, complex compounds are formed by adding potassium iodide or sodium iodide to an aqueous solution of silver nitrate. The present invention further contemplates the use of other metal compounds such as copper iodide, indium trichloride and I zinc iodide in addition to silver nitrate.

In accordance with the first embodiment of the invention, binary alloys of silver with copper, indium or zinc are deposited on an article to be plated. Silvercopper alloy coatings can be obtained by immersing an article to be plated, together with a silver anode, into an iodide-containing solution with a pH value of up to 13.4, or preferably up to 3 and connecting them to a DC power source and passing through the plating solution a direct current of 0.1 to 5 amperes/dm or preferably 0.2 2 amperes/dm The solution should be maintained at a temperature in the range from 15 to C, preferably in the range from 20 to 40C. The aqueous solution comprises 3 4O grams/liter of silver nitrate (preferably, from 6 to 24 grams/liter), l 3O grams/liter of copper iodide (preferably from 2 to 20 grams/liter) and 200 1000 grams/liter of potassium iodide (preferably 300 700 grams/liter). Up to 250 grams/liter (preferably 50 150 grams/liter) of citric acid may advantageously be added. Silver binary alloy coatings with a copper concentration in the range from 1 to 90 atomic percent can be obtained. The copper concentration increases with the amount of current density and decreases with increasing temperature. It has also been found that the copper concentration furthermore depends on the amount of copper salt contained in the solution, and that the silver concentration depends on the amount of silver salt contained in the solution. An x-ray diffraction analysis was made on the so obtained silver-copper alloy coatings and it was found that the alloy of silver and copper forms a solid solution.

According to a second embodiment of the present invention a binary alloy of silver and indium is provided. In this embodiment, the indium concentration in the silver alloy has a tendency to decrease with increasing temperature and decreasing current density. Silverindium alloy coatings with an indium content of from 5 to 30 atomic percent can be obtained by immersing an article into an iodide-containing solution with a pH value of up to 12, preferably up to 3 and then passing a current of 0.1 to 7 amperesldm or preferably 0.2 to 2 amperes/dm through the plating solution from a silver anode. The solution should be maintained in the temperature range from 15 to C, preferably from 20 to 40C. The aqueous solution comprises 3 4O grams/liter of silver nitrate (preferably, from 6 to 24 grams/liter), 4 5O grams/liter of indium trichloride (preferably, from 6 to 30 grams/liter), 110 800 grams/liter of potassium iodide (preferably, from 220 to 600 grams/liter) and up to 250 grams/liter of citric acid (preferably, 50 to 150 grams/liter).

According to a third embodiment of the invention coprecipitation of a binary silver alloy with zinc is attained. In this embodiment, a zinc concentration in the binary alloy of from 8 to 43 atomic percent can be obtained. Silver-zinc alloy coatings can be obtained from an iodide-containing solution with a pH value of up to 13, preferably up to 4 and a current of 0.2 to 5 am peres/dm preferably from, 0.2 to 2.5 amperes/dm The solution should be maintained at a temperature in the range from 15 to 75C, preferably at 20 to 40C. The aqueous solution comprises 3 4O grams/liter of silver nitrate (preferably, from 6 to 24 grams/liter), 1O 120 grams/liter of zinc iodide (preferably, from 20 to grams/liter), 800 grams/liter of potassium iodide (preferably, 220 to 600 grams/liter) and up to 250 grams/liter of citric acid (preferably, 50 to 150 grams/- liter).

A fourth embodiment of the invention discloses deposition of a ternary silver alloy with copper and indium. A silver ternary alloy with a copper concentration of about 14 atomic percent and an indium concentration of about 5 atomic percent can be obtained from an iodide-containing aqueous solution comprising about 8 grams/liter of silver nitrate, about 14 grams/- liter of copper iodide, about 8 grams/liter of indium tr'ichloride, about 500 grams/liter of potassium iodide and up to about 100 grams/liter of citric acid, the solution having a pH value of about 0.6.

According to a fifth embodiment of the invention, a deposit of a ternary silver alloy with copper and zinc is attained. A silver ternary alloy with a copper concentration of about 53 atomic percent and a zinc concentration of about 6 atomic percent can be obtained from an iodide-containing aqueous solution which comprises about 8 grams/liter of silver nitrate, about 14 grams/- liter of copper iodide, about 30 grams/liter of zinc iodide, about 500 grams/liter of potassium iodide and up to about 100 grams/liter of citric acid, the solution having a pH value of about 0.6.

In a sixth embodiment of the invention, deposition of a ternary silver alloy coating with indium and zinc is described. A silver ternary alloy with an indium concentration of about 15 atomic percent and a zinc concentration of about 11 atomic percent can be obtained from an iodide-containing aqueous solution comprising about 8 grams/liter of silver nitrate, about 8 grams/liter of indium trichloride, about 30 grams/liter of zinc iodide, about 500 grams/liter of potassium iodide and up to about 100 grams/liter of citric acid, the solution having a pH value of about 0.6.

According to a seventh embodiment of the invention a quaternary alloy deposit of silver, copper, indium and zinc is obtained. A copper concentration of about 45 atomic percent, an indium concentration of about 10 atomic percent and a zinc concentration of about 4 atomic percent can be deposited using an aqueous iodide solution comprising about 8 grams/liter of silver nitrate, about 14 grams/liter of copper iodide, about 8 grams/liter of indium trichloride, about 30 grams/liter of zinc iodide, about 500 grams/liter of potassium iodide and up to about 100 grams/liter of citric acid, with the solution having a pH value of about 0.6.

The invention will be further described by way of the following examples.

EXAMPLE 1 This Example illustrates codeposition of copper and silver from an aqueous solution having a pH value of 0.6 prepared to contain the following constituents in concentrations indicated:

8 grams/liter l4 grams/liter 500 grams/liter 100 grams/liter ferent temperature conditions. A binary silver alloy with a copper content of from 1 to 90 atomic percent was deposited on the platinum plate under different temperature conditions and current densities, as follows:

Copper Concentration This Example illustrates another example of codeposition of copper and silver from a solution having a pH value of 9.8 prepared by dissolving into water the following constituents in concentrations indicated:

8 grams/liter l4 grams/liter 500 grams/liter Silver nitrate (AgNO;,) Copper iodide (Cul) Potassium iodide (Kl) A similar platinum plate to that used in Example 1 and a silver anode were immersed in the solution and a direct current of 0.3 amperesldm was passed between them with the solution maintained at a temperature of 25C. A silver-copper alloy with a copper content of 25 atomic percent was deposited on the platinum plate.

EXAMPLE 3 Still another example of codeposition of copper and silver is from a solution having a pH value of 0.7 prepared by dissolving into water the following constituents in concentrations indicated:

8 grams/liter 28 grams/liter 500 grams/liter grams/liter Silver nitrate (AgNO Copper iodide (Cul) Potassium iodide (Kl) Citric acid A similar platinum plate to that used in Example 1 and a silver anode were immersed into the solution and subjected to a direct current of 0.3 amperes/dm with the solution maintained at a temperature of 25C. A copper concentration of 53 atomic percent was obtained in the silver-copper alloy coating on the platinum plate.

EXAMPLE 4 Still another example of codeposition of copper and silver is from a solution having a pH value of 0.7 prepared by dissolving into water the following constituents in concentrations indicated:

Silver nitrate (AgNO l6 grams/liter, Copper iodide (Cul) 7 grams/liter Potassium iodide (Kl) 500 grams/liter Citric acid 100 grams/liter A similar platinum plate to that used in Example 1 and a silver anode were immersed into the solution and subjected to a direct current of 0.3 amperes/dm with the solution maintained at a temperature of 25C. A copper concentration in the silver-copper alloy of 3 atomic percent was obtained.

EXAMPLE 5 This Example illustrates codeposition of indium and silver from a solution having a pH value of 12 prepared by dissolving into water the following constituents in the concentrations indicated:

Silver nitrate (AgNO l grams/liter Indium trichloride (lnCl .H O) 8 grams/liter Potassium iodide (Kl) 400 grams/liter Citric acid 100 grams/liter A similar platinum plate to that used in Example 1 and a silver anode were immersed into the solution and subjected to direct currents of from 0.3 to 1.0 ampere/dm with the solution maintained at different temperature conditions. Indium concentrations in the binary silver alloys obtained were as follows:

Indium Concentration Temperature Current Density i th Silv r Alloy (C) (Ampereldm (Atomic EXAMPLE 6 Another example of codeposition of indium and silver is from a solution having a pH value of 1.0 prepared by dissolving into water the following constituents in concentrations indicated:

Silver nitrate (AgNO; l0 grams/liter Indium trichloride (lnCl .H O) 8 grams/liter Potassium iodide (Kl) 400 grams/liter A similar platinum plate to that used in Example 1 and a silver anode were immersed into the solution and subjected to a direct current of 0.5 amperes/dm with the solution maintained at a temperature of 25C. An indium concentration in the silver alloy of atomic percent was obtained.

EXAMPLE 7 Still another example of codeposition of indium and silver is from a solution having a pH value of 1.2 prepared by dissolving into water the following constituents in concentrations indicated:

Silver nitrate (AgNO l0 grams/liter indium trichloride (lnCl .H O) l4 grams/liter Potassium iodide (1(1) 400 grams/liter Citric acid 100 grams/liter A similar platinum plate to that used in Example 1 and a silver anode were immersed into the solution and subjected to a direct current of 0.5 amperes/dm with the solution maintained at a temperature of 25C. An indium concentration in the silver alloy of 25 atomic percent was obtained.

EXAMPLE 8 This Example illustrates codeposition of zinc and silver from a solution having a pH value of 1.2 prepared by dissolving into water the following constituents in concentrations indicated:

8 grams/liter 3O grams/liter 400 grams/liter grams/liter Silver nitrate (AgNO Zinc iodide (Znl Potassium iodide (Kl) Citric acid A similar platinum plate to that used in Example 1 and a silver anode were immersed into the solution and subjected to a direct current of 0.4 to 2.0 amperes/dm with the solution maintained at different temperature conditions. Zinc concentrations in the silver alloy of from 8 to 41 atomic percent were obtained as follows:

Zinc Concentration This illustrates another example of codeposition of zinc and silver from a solution having a pH value of 2.5 prepared by dissolving into water the following constituents in concentrations indicated:

8 grams/liter 30 grams/liter 400 grams/liter Silver nitrate (AgNO- Zinc iodide (Znl Potassium iodide (Kl) A platinum plate similar to that used in Example 1 and a silver anode were immersed into the solution and subjected to direct current of 1.0 ampere/dm with the solution maintained at a temperature of 25C. A zinc concentration in the silver alloy of 20 atomic percent was obtained.

EXAMPLE 10 This Example illustrates another example of codeposition of zinc and silver from a solution having a pH value of 2.5 prepared by dissolving into water the following constituents in concentrations indicated:

Silver nitrate (AgNO-;) 8 grams/liter Zinc iodide (Znl 40 grams/liter Potassium iodide (KI) 400 grams/liter Citric acid 100 grams/liter A platinum plate similar to that used in Example 1 was immersed into the solution and subjected to a current flowing at a density of 1.0 ampere/dm at a temperature of 25C using a silver anode. A zinc concentration of 43 atomic percent was obtained.

EXAMPLE 1 l This Example illustrates codeposition of a ternary alloy of silver, copper and indium from a solution having a pH value of 0.6 prepared by dissolving in water the following constituents in concentrations indicated:

Silver nitrate (AgNO;) 8 grams/liter Copper iodide (Cul) 14 grams/liter lndium trichloride (lnCl- .H O) 8 grams/liter Potassium iodide (Kl) 500 grams/liter Citric acid 100 grams/liter A platinum plate similar to that used in Example 1 was immersed into the solution and subjected to a current flowing at a density of 0.3 amperes/dm at a temperature of 25C using a silver anode. The copper and indium concentrations in the silver alloy were 14 and 5 atomic percent, respectively.

EXAMPLE 12 This Example illustrates codeposition of a ternary alloy in which silver, copper and zinc are coprecipitated from a solution having the same pH value as in Example 11 and containing the following constituents in concentrations indicated:

Silver nitrate (AgNO;) 8 grams/liter Copper iodide (Cul) l4 grams/liter Zinc iodide (Znl 30 grams/liter Potassium iodide (Kl) Citric acid 500 grams/liter 100 grams/liter A platinum plate similar to that used in Example 1 was immersed into the solution and subjected to an electric current flowing at a density of 0.4 amperes/dm at a temperature of 25C using a silver anode. The copper and zinc concentrations in the silver alloy were 53 and 6 atomic percent, respectively.

EXAMPLE l3 Illustrated here is another example of codeposition of a ternary alloy in which silver, indium and zinc are 00- precipitated from a solution having the same pH value as in Example 1 l and containing the following constituents in concentrations indicated:

Silver nitrate (AgNO;,) 8 grams/liter lndium trichloride (lnCl .H O) 8 grams/liter Zinc iodide (Znl- 3O grams/liter Potassium iodide (Kl) 500 grams/liter Citric acid 100 grams/liter A platinum plate similar to that used in Example 1 was immersed into the solution and subjected to an electric current flowing at a density of 0.5 amperes/dm at a temperature of 25C using a silver anode. The indium and zinc concentrations in the silver alloy were and l 1 atomic percent, respectively.

EXAMPLE 14 This Example illustrates codeposition of a quaternary alloy of silver, copper, indium and zinc from a solution having the same pH value as in Example 11 and containing the following constituents in concentrations indicated:

Silver nitrate (AgNO;,) 8 grams/liter Copper iodide (Cul) l4 grams/liter lndium trichloride (lnCl .H O) 8 grams/liter 3O grams/liter 500 grams/liter 100 grams/liter Zinc iodide (Znl Potassium iodide (Kl) Citric acid A platinum plate similar to that used in Example 1 was immersed into the solution and subjected to an electric current flowing at a density of 0.4 amperes/dm at a temperature of 25C using a silver anode. The respective concentrations in the silver alloy were copper 45 atomic percent, indium 10 atomic percent, and zinc 4 atomic percent.

EXPERIMENT To determine the tarnish-free properties of the silver alloy deposits, humidity and hydrogen sulphide tests were conducted. For comparison purposes a silver alloy was prepared by a known method using a silver cyanide bath. The silver cyanide bath was prepared by dissolving into water the following constituents in concentrations indicated:

36 grams/ liter Silver cyanide (AgCN) 60 grams/liter Potassium cyanide (KCN) Potassium carbonate z a) 26 grams/liter A piece of copper plate 50 microns thick was immersed into the solution which was maintained at a temperature of 25C and subjected to a current of 1.0 amperes/dm across the copper plate and a silver anode. The silver coating deposited on the copper plate had a thickness of from 2 to 3 microns;

Next, various test samples were prepared by immersing pieces of copper plate 50 microns thick into respective iodide solutions. The aqueous solutions comprised the following constituents in concentrations indicated:

Sample solution A pH 0.6 (Silver-copper alloy) Silver nitrate (AgNo Copper iodide (Cul) Potassium iodide (Kl) Citric acid Sample solution B pH 1.2 (Silver-indium alloy) 8 grams/liter l4 grams/liter 500 grams/liter 100 grams/liter Silver nitrate (AgNO 10 grams/liter Indium trichloride (InCl .H O) 8 grams/liter Potassium iodide (Kl) 400 grams/liter Citric acid 100 grams/liter Sample solution C pH 1.2 (Silver-zinc alloy) Silver nitrate (AgNO 8 grams/liter Zinc iodide (Znl 30 grams/liter Potassium iodide (Kl) 400 grams/liter Citric acid 100 grams/liter The following silver alloy samples were prepared under the following conditions.

pieces in an atmosphere of humid air of from to relative humidity and maintaining them therein for a period of 1000 hours at a temperature of 40C. A sample piece prepared by the cyanide bath method started to change its color to brown after 250 hours, the color deepening with subsequent exposure. On the other hand, sample piece A (silver-copper alloy) exhibited only a slight color change after 600 hours. Sample piece B (silver-indium alloy) started to change its color after 800 hours. Sample pieces C and D (silver-indium) did not exhibit any color change even after 1000 hours exposure. Likewise, sample pieces F and G (silver-zinc alloys) did not exhibit any color change after the lOOO- hour test although sample piece E (silver-Zinc) exhibited some color change after 800 hours.

HYDROGEN SULPHIDE TESTS Hydrogen sulphide tests were conducted by placing the samples in a desiccator containing a substance which contains hydrogen sulphide, the desiccator being maintained at a temperature of 60C and at a relative humidity of from 90 to 95%. The samples were maintained in the deciccator for a period of 10 hours. The tests revealed that the sample prepared by the known cyanide method changed its color to black while Samples A to G did not exhibit any color change.

RESULTS OF MEASUREMENTS proximately 8 atomic percent which is substantially in agreement with the maximum copper content of metallurgically produced silver-copper alloy.

What is claimed is:

1. An aqueous electroplating bath for the deposition of an alloy of silver and copper, comprising 6 to 24 grams per liter of silver nitrate, 300 to 700 grams per liter of potassium iodide and 2 to grams per liter of copper iodide, the pH value of the bath being not more than 3.0.

2. An electroplating bath according to claim 1, further comprising 50 to 150 grams per liter of citric acid.

3. An aqueous electroplating bath for the deposition of an alloy of silver and indium, comprising 6 to 24 grams per liter of silver nitrate, 220 to 600 grams per liter of potassium iodide and 6 to 30 grams per liter of indium trichloride, the pH value of the bath being not more than 3.0.

4. An electroplating bath according to claim 3, further comprising 50 to 150 grams per liter of citric acid.

5. An aqueous electroplating bath for the deposition of an alloy of silver and zinc, comprising 6 to 24 grams per liter of silver nitrate, 220 to 600 grams per liter of potassium iodide and 20 to 80 grams per liter of zinc iodide, the pH value of the bath being not more than It Is generally recognized that in metallurgy the alloy 4 O of silver and copper is able to form a solid solution with An electroplating bath according to claim 5 9 a coplier if g i up to 2% 8 i ther comprising 50 to l50 grams per liter of citric acid. g i )Hay l ractlon an t e 7. An aqueous electroplating bath for the deposition raggs Ormu a expresse y of an alloy of silver, copper and indium, comprising "A 2 i 9 about 8 grams per liter of silver nitrate, about 500 grams per liter of potassium iodide, about 14 grams per i g k f i .g liter of copper iodide, about 8 grams per liter of indium crysta Ograp i p anes an ang.e 0 mm .ence 9 trichloride and about 100 grams per liter of citric acid, the x-ray relative to a crystallographic plane. Since 511- the P value of the bath being about 0 6 forms a i the lame? Constant deter 8. An aqueous electroplating bath for the deposition mmed by the Mmer mihces an z blstar-lce of an alloy of silver, copper and zinc, comprising about f x ciystzznogriphlc Planes 15 etermme y usmg 8 grams per liter of silver nitrate about 500 grams per t e 0 Owmg 0mm liter of potassium iodide, about 14 grams per liter of a d x V 17 k 1 copper iodide, about 30 grams per liter of zinc iodide where, a is the lattice constant. Since thelattice con- 2; 3 g am g g 2 clmc the P Stan: varies in proportionbtoddistance, 5h; variation of g ll gqseoi s elsgt foglgigg Ba-th for the deposition. the attice constant can e etermlne y measuring the angle of incidence of the x-ray. If the lattice con- 3 an alloy l i tg q i i i s gg about stant has been varied it can be said that a SOllCl solution Pf er 0 te a Cu lg zamsfper was formed in the alloy 1.61 0 p0 assium m ra e, a on grams per 1 up n- Measurements were made on a silvepcopper deposit dlum trichloride, about 30 grams per liter of zinc iodide, obtained from the solution of Example 1, (with the sol 2 & fi g l P g g 2 Cltrlc 391d, the P lution being maintained at a temperature of 25C) Va 0 t e at g a out using an X-ray diffractometer (PW-1009), a Cuk 10. An aqueous electroplating bath for the deposition get and a nickel filter. The measurements yielded the Of an l y of Silver, pp indium and Zinc, pri following results: ing about 8 grams per liter of silver nitrate, about 500 Plane 29 eg Copper Orientation Current W Content (h,k,l) (Amp/dm) Average (A) (A) (Atomic The above results were obtained by assuming that n grams per liter of potassium iodide, about 14 grams per 1, A 1.5405 A, a, 4.0862 A, a 3.6150 A (from liter of copper iodide, about 8 grams per liter of indium X-ray-Powder Data File of ASTM) and atomic concentration (a CI|)/( ,4y m)- The copper concentration in the alloy deposit is aptrichloride, about 30 grams per liter of zinc iodide and about 100 grams per liter of citric acid, the pH value of the bath being about 0.6.

11. A method of forming a deposit of an alloy of silver and copper by electroplating an article in an electroplating bath as claimed in claim 1, comprising passing a DC current through said bath at a current density of from 0.2 to 2 amperes/dm and at a bath temperature of to 40C.

12. A method of forming a deposit of an alloy of silver and indium by electroplating an article in an electroplating bath as claimed in claim 3, comprising passing a DC current through said bath at a current density 

1. AN AQUEOUS ELECTROPLATING BATH FOR THE DEPOSITION OF AN ALLOY OF SILVER AND COPPER, COMPRISING 6 TO 24 GRAMS PER LITER OF SILVER NITRATE, 300 TO 700 GRAMS PER LITER OF POTASSIUM IODIDE AND 2 TO 20 GRAMS PER LITER OF COPPER IODIDE, THE PH VALUE OF THE BATH BEING NOT MORE THAN 3.0.
 2. An electroplating bath according to claim 1, further comprising 50 to 150 grams per liter of citric acid.
 3. AN AQUEOUS ELECTROPLATING BATH FOR THE DEPOSITION OF AN ALLOY OF SILVER AND INDIUM, COMPRISING 6 TO 24 GRAMS PER LITER OF SILVER NITRATE, 220 TO 600 GRAMS PER LITER OF POTASSIUM IODIDE AND 6 TO 30 GRAMS PER LITER OF INDIUM TRICHLORIDE, THE PH VALUE OF THE BATH BEING NOT MORE THAN 3.0.
 4. An electroplating bath according to claim 3, further comprising 50 to 150 grams per liter of citric acid.
 5. AN AQUEOUS ELECTROPLATING BATH FOR THE DEPOSITION OF AN ALLOY OF SILVER AND ZINC, COMPRISING 6 TO 24 GRAMS PER LITER OF SILVER NITRATE, 220 TO 600 GRAMS PER LITER OF POTASSIUM IODIDE AND 20 TO 80 GRAMS PER LITER OF ZINC IODIDE, THE PH VALUE OF THE BATH BEING NOT MORE THAN 4.0.
 6. An electroplating bath according to claim 5, further comprising 50 to 150 grams per liter of citric acid.
 7. AN AQUEOUS ELECTROPLATING BATH FOR THE DEPOSITION OF AN ALLOY OF SILVER, COPPER AND INDIUM, COMPRISING ABOUT 8 GRAMS PER LITER OF SILVER NITRATE, ABOUT 500 GRAMS PER LITER OF POTASSIUM IODIDE, ABOUT 14 GRAMS PER LITER OF COPPER IODIDE, ABOUT 8 GRAMS PER LITER OF INDIUM TRICHLORIDE AND ABOUT 100 GRAMS PER LITER OF CITRIC ACID, THE PH VALUE OF THE BATH BEING ABOUT 0.6.
 8. AN AQUEOUS ELECTROPLATING BATH FOR THE DEPOSITION OF AN ALLOY OF SILVER, COPPER AND ZINC, COMPRISING ABOUT 8 GRAMS PER LITER OF SILVER NITRATE, ABOUT 500 GRAMS PER LITER OF POTASSIUM IODIDE, ABOUT 14 GRAMS PER LITER OF COPPER IODIDE, ABOUT 30 GRAMS PER LITER OF ZINC IODIDE AND ABOUT 100 GRAMS PER LITER OF CITRIC ACID, THE PH VALUE OF THE BATH BEING ABOUT 0.6.
 9. An aqueous electroplating bath for the deposition of an alloy of silver, Indium and zinc, comprising about 8 grams per liter of silver nitrate, about 500 grams per liter of potassium nitrate, about 8 grams per liter of indium trichloride, about 30 grams per liter of zinc iodide and about 100 grams per liter of citric acid, the pH value of the bath being about 0.6.
 10. An aqueous electroplating bath for the deposition of an alloy of silver, copper, indium and zinc, comprising about 8 grams per liter of silver nitrate, about 500 grams per liter of potassium iodide, about 14 grams per liter of copper iodide, about 8 grams per liter of indium trichloride, about 30 grams per liter of zinc iodide and about 100 grams per liter of citric acid, the pH value of the bath being about 0.6.
 11. A method of forming a deposit of an alloy of silver and copper by electroplating an article in an electroplating bath as claimed in claim 1, comprising passing a DC current through said bath at a current density of from 0.2 to 2 amperes/dm2 and at a bath temperature of 20* to 40*C.
 12. A method of forming a deposit of an alloy of silver and indium by electroplating an article in an electroplating bath as claimed in claim 3, comprising passing a DC current through said bath at a current density of from 0.2 to 2 amperes/dm2 and at a bath temperature of 20* to 40*C.
 13. A method of forming a deposit of an alloy of silver and zinc by electroplating an article in an electroplating bath as claimed in claim 5, comprising passing a DC current through said bath at a current density of from 0.2 to 25 amperes/dm2 and at a temperature of 20* to 40*C. 